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Barnard's Star b
An artist’s conception shows what the surface of the reported planet known as Barnard’s Star b might look like. (ESO Illustration / M. Kornmesser)

The astronomical team that found the nearest exoplanet at Proxima Centauri has done it again with the reported detection of a super-Earth orbiting Barnard’s Star, the second-closest star system to our own.

The discoverers acknowledge, however, that they’re not completely sure yet.

“After a very careful analysis, we are 99 percent confident that the planet is there,” Spanish astronomer Ignasi Ribas, lead author of a study about the detection published today by the journal Nature, said in a news release. “However, we’ll continue to observe this fast-moving star to exclude possible, but improbable, natural variations of the stellar brightness which could masquerade as a planet.”

Assuming it exists, Barnard’s Star b would be at least 3.2 times as massive as Earth, tracing a 233-Earth-day orbit. It would be as close to its parent star as Mercury is to our own sun — but because Barnard’s Star is a dim red dwarf, surface conditions would be far too chilly for life as we know it. The surface temperature would be about 275 degrees below zero Fahrenheit (-170 degrees Celsius).

The dimness of Barnard’s Star also explains the difficulty and the slight uncertainty surrounding the detection. Astronomers had to combine nearly 800 observations from seven different instruments, spanning 18 years’ worth of archives, to trace faint variations in the spectral characteristics of starlight from Barnard’s star.

Patterns in those spectral variations can point to the gravitational wobbles induced by a planet orbiting an alien star. This type of analysis, known as the radial velocity method, led to the very first detections of extrasolar planets in the mid-1990s. More recently, most exoplanets have been detected using a different technique known as the transit method.

Ribas, who is the director of the Institute of Space Studies of Catalonia and a research at Spain’s Institute of Space Sciences, noted that there have been many previous searches for planets around Barnard’s Star, and even announcements of discoveries.

“Not one has ever been confirmed,” he said in a Q&A published by the European Southern Observatory. “The thing is that the candidate planet we found is so small and so far from its host star that its effect on the star is really, really tiny.”

The key observations came from the High Accuracy Radial Velocity Planet Searcher on the ESO’s 3.6-meter La Silla telescope in Chile, also known as HARPs, plus the CARMENES instrument at the Calar Alto Observatory in southern Spain.

Despite this particular planet’s seeming inhabitability, the reported detection raises hopes that astronomers could get a closeup look at the type of exoplanet considered most likely to have conditions conducive to life.

In a Nature commentary, University of Buenos Aires astronomer Rodrigo Diaz said the next generation of telescopes should be able to capture direct images of the planet and measure its light spectrum.

“Using this spectrum, the characteristics of the planet’s atmosphere — such as its winds and rotation rate — could be inferred,” Diaz wrote. “This remarkable planet therefore gives us a key piece in the puzzle of planetary formation and evolution, and might be among the first low-mass exoplanets whose atmospheres are probed in detail.”

Ribas and his colleagues are part of the Red Dots collaboration, which is surveying a wide swath of red dwarfs for evidence of planets.

The collaboration announced its first big find, Proxima Centauri b, in 2016. Proxima b is 4.2 light-years from Earth is at least 30 percent more massive than Earth. It might be potentially habitable, although there are lots of unanswered questions about that.

Rocky super-Earths are thought to be among the best candidates for life beyond our solar system. Thus, closer studies of Proxima b, Barnard’s Star b and other super-Earths should help scientists come closer to figuring out whether the conditions for life are rare or common in the universe.

“Much of the universe is still a complete mystery,” Ribas said. “At the moment we are exploring it long-distance, from Earth, but perhaps someday in the distant future we will really be able to visit these planets, so we need to find out more about them first.”

Ribas is among more than 60 authors of the study published in Nature, “A Super-Earth Planet Candidate Orbiting at the Snow-line of Barnard’s Star.” Teams of semi-professional astronomers coordinated by the American Association of Variable Star Observers also contributed to the detection.

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